Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and other content. These systems may be multiple-access systems capable of simultaneously supporting communication for multiple wireless terminals by sharing the available transmission resources (e.g., frequency channel and/or time interval). Since the transmission resources are shared, efficient allocation of the transmission resources is important as it impacts the utilization of the transmission resources and the quality of service perceived by individual terminal users. One such wireless communications system is the Orthogonal Frequency-Division Multiple Access (OFDMA) system in which multiple wireless terminals perform multiple-access using Orthogonal Frequency-Division Multiplexing (OFDM).
OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple orthogonal frequency subchannels, each of which is associated with a respective subcarrier that may be modulated with data. Because the subchannels are made orthogonal, some spectral overlap between the subchannels is permitted, leading to a high spectral efficiency. In OFDM systems, the user data stream is split into parallel streams of reduced rate, and each obtained substream then modulates a separate subcarrier.
In OFDMA, the transmission resource extends over two dimensions: frequency channels and time intervals. The resources of a given frequency channel may involve contiguous and/or non-contiguous groups of subcarriers.
Examples of OFDM communication systems include, but are not limited to, wireless protocols such as the wireless local area network (“WLAN”) protocol defined according to the Institute of Electrical and Electronics Engineering (“IEEE”) standards radio 802.11a, b, g, and n (hereinafter “Wi-Fi”), the Wireless MAN/Fixed broadband wireless access (“BWA”) standard defined according to IEEE 802.16 (hereinafter “WiMAX”), the mobile broadband 3GPP Long Term Evolution (“LTE”) protocol having air interface High Speed OFDM Packet Access (“HSOPA”) or Evolved UMTS Terrestrial Radio Access (“E-UTRA”), the 3GPP2 Ultra Mobile Broadband (“UMB”) protocol, digital radio systems Digital Audio Broadcasting (“DAB”) protocol, Hybrid Digital (“HD”) Radio, the terrestrial digital TV system Digital Video Broadcasting-Terrestrial (“DVB-T”), the cellular communication systems Flash-OFDM, etc. Wired protocols using OFDM techniques include Asymmetric Digital Subscriber Line (“ADSL”) and Very High Bitrate Digital Subscriber Line (“VDSL”) broadband access, Power line communication (“PLC”) including Broadband over Power Lines (“BPL”), and Multimedia over Coax Alliance (“MoCA”) home networking.
Generally, in OFDMA systems each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink (DL)) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink (UL)) refers to the communication link from the terminals to the base stations.
In 3GPP LTE, the following physical channels are defined:
Downlink (DL)                Physical Broadcast Channel (PBCH): This channel carries system information for user equipments (UEs) requiring access to the network.        Physical Downlink Control Channel (PDCCH): The main purpose of this physical channel is to carry scheduling information.        Physical Hybrid ARQ Indicator Channel (PHICH): This channel is used to report the Hybrid ARQ status.        Physical Downlink Shared Channel (PDSCH): This channel is used for unicast and paging functions.        Physical Multicast Channel (PMCH): This physical channel carries system information for multicast purposes.        Physical Control Format Indicator Channel (PCFICH): This channel provides information to enable the UEs to decode the PDSCH.        
Uplink (UL)                Physical Uplink Control Channel (PUCCH): This channel is used to transport user signaling data from one or more UE that can transmit on the control channel. The PUCCH transports, for example, acknowledgment responses and retransmission requests, service scheduling requests, and channel quality information measured by the UE to the system.        Physical Uplink Shared Channel (PUSCH): This channel is used to transport user data from one or more mobiles that can transmit on the shared channel.        Physical Random Access Channel (PRACH): This uplink physical channel allows a mobile to randomly transmit access requests when the mobile attempts to access the wireless communication system.        
The DL and UL communication links in a wireless multiple-access communication system may be established via one antenna at the transmitter and one antenna at the receiver (single-input-single-output, or SISO), via multiple antennas at the transmitter and one antenna at the receiver (multiple-input-single-output, or MISO), via one antenna at the transmitter and multiple antennas at the receiver (single-input-multiple-output, or SIMO), or via multiple antennas at the transmitter and multiple antennas at the receiver (multiple-input-multiple-output, or MIMO).
A MIMO system may employ transmit diversity to combat the effect of fast fading by using multiple antennas to transmit a data stream via multiple independently fading channels. Transmit diversity schemes can be divided into open loop transmit diversity (OLTD) and closed-loop transmission diversity (CLTD) schemes. In OLTD, no feedback is required from the receiver. In one type of CLTD, a receiver knows an arrangement of transmission antennas, and computes a phase and amplitude adjustment that should be applied at the transmitter antennas in order to maximize a power of the signal received at the receiver. In another type of CLTD, referred to as selection transmit diversity (STD), the receiver provides feedback information to the transmitter regarding which antenna(s) to be used for transmission.
An example OLTD scheme is the Alamouti 2×1 space-time diversity scheme. The Alamouti 2×1 space-time diversity scheme contemplates transmitting a Alamouti 2×2 block code using two transmission antennas using either two time slots (i.e., Space-Time Block Code (STBC) transmit diversity) or two frequency subcarriers (i.e., Space-Frequency Block Code (SFBC) transmit diversity).
A major problem with the multi-carrier modulation in general and OFDM communication systems in particular is the high peak-to-average power ratio (PAPR) that is inherent in the transmitted signal. Large signal peaks occur in the transmitted signal when the signals in the subcarriers add constructively in phase. Such large signal peaks may saturate the power amplifier (PA) at the transmitter and thus, cause nonlinear distortion of the transmitted signal, which results in a large degradation of performance, e.g. increase of both the bit error rate (BER) and the out-of-band radiation (spectral spreading). This high PAPR problem may be partially overcome in DL transmission by utilizing more advanced PAs with larger dynamic ranges. However, when it comes to UL transmission, the restrictions of the user equipment (UE), in terms of price and dimensions, precludes the possibility of this solution.
In 3GPP Release 8 E-UTRA, in which only one transmit antenna is supported at the UE, two separate methods have been utilized for PUCCH and PUSCH, respectively, to keep the PAPR as low as possible. In PUCCH, where code-division multiple access (CDMA) is the multiple-access method, orthogonal spreading codes are designed such that they provide a relatively low PAPR at the output of the Inverse Discrete Fourier Transform (IDFT) signal processing step of OFDM. In PUSCH, on the other hand, Single Carrier Frequency-Division Multiple Access (SC-FDMA) has been adopted as the multiple-access scheme to decrease the PAPR as compared with OFDMA. SC-FDMA is a modulation and multiple-access scheme which, due to its inherent single carrier structure, has a lower transmit signal PAPR than OFDMA.
In Advanced E-UTRA, increased peak data rates (e.g. up to 500 Mbps in the UL) are targeted. A promising technique to fulfil these high data rates is MIMO. In cases where MIMO is used, the UE can utilize an OLTD scheme to support the target data rates at acceptable error rates. However, the OLTD schemes currently proposed suffer either from the high PAPR problem described above or from what is known in the art as the orphan symbol problem. For example, STBC preserves the low PAPR property but requires an even number of symbols per slot, whereas SFBC works for any number of symbols but increases the PAPR. Cyclic Delay Diversity (CDD), another candidate transmit diversity scheme, preserves the low PAPR property and works for any number of symbols, but suffers from poorer performance relative to STBC and SFBC.
Another problem that arises due to MIMO relates to UL channel estimation. UL reference signals (RS) in LTE can be classified into three broad types: reference signals for demodulation of PUSCH, reference signals for demodulation of PUCCH, and reference signals for measurement of UL channel quality. Currently, for certain PUCCH formats in LTE, RS symbols are separated from each other within each slot. This RS separation is not beneficial in low SNR, because interpolation accuracy is degraded in low SNR. A straightforward solution is to assign two orthogonal sequences (OS) to each UE so that the channel estimation for each antenna can be performed as in the single antenna case. However, this solution wastes resources (orthogonal sequences) which could otherwise be used to support more UEs. Moreover, this solution entails some signalling overhead to inform the UEs which additional sequence to choose.
A need exists for improved transmit diversity schemes for uplink transmission. A need also exists for improved channel estimation schemes.